Marine Corps Gen. James Mattis, head of the U.S. Central Command. / Alex Brandon, AP

by Tom Vanden Brook, USA TODAY

by Tom Vanden Brook, USA TODAY

WASHINGTON -- Afghan contractors responsible for preventing culverts from being used to hide roadside bombs on a major highway have falsely reported completing the work, putting American troops at risk, U.S. investigators revealed Thursday.

It's unclear if any U.S. troops have been killed or wounded because of the potential fraud, and a criminal investigation is underway. The announcement comes as U.S. troops have increasingly been targeted for attack by Afghan security forces.

"We've heard this tune again and again for the last 10 years," said Peter Singer, director of the 21st Century Defense Initiative at the Brookings Institution. "Another sad illustration of how corruption in the realm of contracting not only led to lost taxpayer money but also potential lost lives."

A Navy contract officer first reported potential fraud involving work on the culverts, the U.S. command in Kabul said in a statement. The contractors were paid $361,680 to place 125 metal grates over culverts to prevent insurgents from packing them with improvised explosive devices (IEDs), the top cause of American casualties in Afghanistan.

"Given the increased risk of IED attack against U.S. forces resulting from the missing or defective culvert denial systems, we are providing this information to you for immediate action and dissemination to all relevant personnel," John Sopko, the special inspector general for Afghanistan reconstruction, wrote in a letter to Marine Gen. James Mattis, the head of U.S. Central Command, which oversees U.S. troops in the country.

Lt. Gen. Michael Barbero, director of the Pentagon's Joint IED Defeat Organization, said there "are obvious concerns with IEDs hidden in culverts" and several of the culvert-protection systems have been effective. "While JIEDDO was not involved in the funding or acquisition of these specific culvert protection systems in question, we remain concerned for the security of our troops from any IED threat."

The problem was first reported in August in one region of the country. That location was redacted in the letter released Thursday.

"However, we are concerned that this problem may be more widely spread throughout Afghanistan," Sopko told Mattis in the letter.

Makeshift bombs account for 60% of deaths and injuries in Afghanistan, according to JIEDDO. From July through September, insurgents in Afghanistan, planted 4,346 bombs, a decline of 12% for the same period last year, it says.

The bombs killed or wounded 669 U.S. troops during that period, down from 1,356 a year ago. One reason for the decline is the rate at which troops find bombs before they explode. Last year, troops riding in vehicles detected just over half the bombs before they blew up. Now they are finding two-thirds. Troops on foot do even better: finding nearly four in five bombs before they detonate, a slight improvement over last year's rate.